The present invention relates to an xcex1-lithium aluminate (LiAlO2) which may be used advantageously as an electrolyte supporting plate of a molten carbonate-type fuel cell (MCFC), and the method of manufacturing the same.
An electrolyte supporting plate of an MCFC is used to support a mixed molten carbonate such as lithium carbonate (Li2CO3), potassium carbonate (K2CO3), sodium carbonate (Na2CO3) and the like, under a high temperature range around 650xc2x0 C. Therefore, the material for such supporting plate is required to have properties such as high support ability to molten carbonate, stability of particle shape, alkali resistance, heat resistance and the like. As a material satisfying such requirements, a lithium aluminate is used conventionally as the material forming the electrolyte supporting plate. Especially, a fine xcex3-lithium aluminate with a relatively large specific surface area and having an advantageous electrolyte supporting power is utilized.
Many methods for manufacturing the xcex3-lithium aluminate with a large specific surface area are disclosed, for example in Japanese Laid-Open Patent Publications 60-65719, 60-151975, 61-295227, 61-295228, 63-270311, 1-252522, 2-80319and the like. These known methods mainly characterize in that a mixture of alumina (Al2O3) and lithium hydroxide (LiOH) or lithium carbonate is baked under a temperature in the range of 600-1000xc2x0 C. so as to restrain the fining of the structure, or to increase the specific surface area of the material by providing a secondary process forming a porous structure or by providing a hydration process.
Recently, it has been discovered that an xcex1-lithium aluminate is also useful as an electrolyte supporting plate in an MCFC.
Japanese Laid-Open Patent Publication 2-243511 discloses a method of manufacturing a high crystalline xcex1-lithium aluminate.
Moreover, Japanese Laid-Open Patent Publication 9-97620 discloses a method of heat-treating a mixture of carbonate including lithium carbonate and alumina powder having a specific surface area of over 100 m2/g in molten carbonate heated to 700-800xc2x0 C.
Even further, Japanese Laid-Open Patent Publication 10-112329 discloses a method of heat-treating a mixture of carbonate including lithium carbonate and aluminum hydroxide powder having a specific surface area of over 100 m2/g in molten carbonate heated to 700-800xc2x0 C.
The above methods all involve heat-treating alumina or lithium aluminate material in molten carbonate. Such methods require long reaction time ranging from 50 hours to 100 hours. Further, the methods inevitably require a step of washing and drying the product for removal of carbonate, which complicates the manufacturing steps and increases the cost.
As mentioned above, the prior art of manufacturing an xcex1-lithium aluminate does not fully respond to the needs for an advanced supporting material which realizes a longer lifetime of the MCFC, which includes advanced supportability of molten carbonate, and improved alkali resistance and heat resistance.
In order to solve the problems of the prior art, the present inventors have discovered through studies that an xcex1-lithium aluminate obtained by mixing a porous xcex3-alumina and a lithium compound so that the Li/Al mol ratio is set approximate the stoichiometry ratio and baking said material has improved properties in that the particle structure of the product does not change even when being heated for long hours in molten carbonate, and that the product provides advanced alkali resistance, heat resistance and very high level of supportability.
The present invention is completed based on the above discovery. The object of the invention is to provide an xcex1-lithium aluminate guaranteeing advanced thermal stability and chemical stability in molten carbonate which is especially suited as the material for the electrolyte supporting plate in an MCFC, and to provide a method of manufacturing said xcex1-lithium aluminate having stable properties in molten carbonate by steps of simply dry-mixing the material powder and baking the same, which involves no complicated manufacturing steps.
The present invention relates to an xcex1-lithium aluminate obtained through solid-phase reaction of a xcex3-alumina and a lithium compound, wherein said xcex1-lithium aluminate comprises particles having a round shape, with a BET specific surface area in the range of 4-7 m2/g, and substantially not including xcex3-lithium aluminate.
Moreover, the present invention relates to an xcex1-lithium aluminate disclosed above, wherein an average particle size (C2) measured by a laser method is in the range of 0.5-2.0 xcexcm, and the ratio (D=C2/C1) of said average particle size (C2) to a particle size (C1) calculated through spherical conversion based on said BET specific surface area is in the range of 2-4.
Even further, the present invention relates to a method of manufacturing an xcex1-lithium aluminate, including the steps of dry-mixing a xcex3-alumina having a BET specific surface area in the range of 70-140 m2/g and a lithium compound, and then baking and reacting said mixture in a solid state.
The present invention further relates to a method of manufacturing an xcex1-lithium aluminate disclosed above, wherein a xcex3-alumina having an average particle size in the range of 0.7-2.0 xcexcm measured by a laser method is utilized as a starting material. Moreover, the present invention relates to a method of manufacturing an xcex1-lithium aluminate disclosed above, wherein the mol ratio (Li/Al) of said xcex3-alumina to said lithium compound is adjusted to 0.95-1.05 when dry-mixing, before performing said baking process.
Even further, the present invention relates to a method of manufacturing an xcex1-lithium aluminate disclosed above, wherein the ratio (Y=X2/X1) of the average particle size (X1) of the material xcex3-alumina measured by a laser method to the average particle size (X2) of the produced xcex1-lithium aluminate measured by a laser method is in the range of 0.7-1.0.
The present invention also relates to an electrolyte support material of a molten carbonate fuel cell formed of said xcex1-lithium aluminate defined above.